PLease help re cashback

I signed up with Plusnet and signed for a package with cashback. The contract stated i would receive the cashback email within 10 days of activation, its now long past that time. I have written numerous emails etc to plusnet customer services, and spent ages with different staff members on the phone. Despite all this i have been offered no reason for their failure to give me the cashback as stated in my contract with them. kgmck